This invention relates to a clamping device for dental handpieces.
More particularly, the invention is concerned with dental handpieces having an elongated handpiece sleeve in which a drive shaft is rotatably mounted. The drive shaft drives a tooth treatment tool e.g. a drill. The drive shaft has at one end an opening or hollow portion at its output side to receive the tool shank. The drive shaft at its output side is provided with several depressions distributed over the circumferential wall of the cavity of the end portion, of decreasing depth as viewed in radial section. These depressions are provided for the external suspension of roll bodies which form a roll clamping lock retaining the tool during rotation of the driving shaft. The internal suspension of the roll bodies are formed by openings in a bushing which surrounds the tool shank and is formed in the manner of a roll or antifriction body cage. The openings permit application of or abutment of the roll bodies against the tool shank. The bushing is provided with a pre-clamping element to retain the inserted tool prior to the starting of the rotation of the driving shaft.
In principle, the roll bodies of such self-locking roll clamping locks may be balls. Expediently, however, the roll bodies consist of needles or rollers. The depressions serve for the external suspension for the roll bodies and are designed as axial grooves; and the openings which provide for the internal suspension are axial slots. The depth of the depressions varies in an arcuate fashion, particularly as seen in radial section, and expediently decreases uniformly from the lowest point toward both sides so that the roll bodies occupy their clamping position relative to the tool shank independently of the direction of rotation of the driving shaft when the latter starts up. As a result of a non-uniform decrease of the depth of the depressions a freewheeling can be obtaining in one of the two directions of rotation of the driving shaft. The number of roll bodies or roll elements is generally more than two, preferably three.
The terms used "axial" and "radial" refer to the axes of the driving shaft or respectively to the handpiece sleeve.
A clamping device having the aforesaid features is known from German Pat. No. 811,250. In this known prior art clamping device, a pre-clamping element forming part of a bushing is used to retain or hold the tool prior to the starting of the rotation of the drive shaft. For this purpose, springs are placed around the bushing and act on the roll bodies which consist of needles centrically, i.e. radially inward. A similar arrangement with such springs is disclosed in German Pat. No. 592,225. Upon starting of the drive shaft, the springs constitute a counter-force which act on the roll bodies; the entire control is based upon the proper dimensioning of the springs, and this is usually quite difficult. Accordingly, depending on the dimensioning of the springs, the roll bodies either occupy their clamping position or are prevented from doing so. If the spring force is too weak, the roll bodies are permited to occupy the clamping position, but the roll bodies are prevented from moving out of the clamping position because of the now additionally generated clamping force of the springs. The correct dimensioning of the springs is therefore difficult.
A clamping device similar to the aforementioned type, but without a bushing having a pre-clamping element which surrounds the tool shank, i.e. arranged in the cavity of the driving shaft, is disclosed in German Pat. No. 905,528. In this known clamping device, the pre-clamping element consists of a separate arrangement. In this arrangement, three clamping balls are disposed around the tool shank which are provided in openings in the driving shaft and are arranged in the manner of a ball bearing cage so that they can be applied internally against the tool shank and externally against the conical inner wall of a clamping sleeve in clamped position under the action of a clamping spring and mounted on the driving shaft. By means of an outer grip, the clamping sleeve can be displaced on the driving shaft counter to the action of the clamping spring axially into the release position. In this known clamping device, inwardly pressing the clamping balls may cause damage and deformations on the tool shank which will make repeated use of the tool and its extraction from the clamping device doubtful.
Moreover, all mentioned known clamping devices have the following disadvantages: When a strong load is applied to the tooth treatment tool, i.e. at increased torque, the roll bodies may be wedged between the driving shaft and tool shank so strongly that at decreased torque or standstill of the driving shaft the roll bodies will no longer be able to become released from their clamping position. As a result, the tool cannot be extracted from the clamping device and hence from the handpiece. If, in order to prevent this, the depressions distributed over the inner circumferential wall of the hollow end portion of the driving shaft were formed with a depth decreasing more as seen in radial section, then, difficulties would arise under decreased load at too great a decrease of the depth, the clamping force of the roll bodies would, during the decrease of the load of the tool, i.e. at reduction of the torque, diminish while the driving shaft is still running; or respectively, the roll bodies would release from their clamping position, so that the tool would start to vibrate and become released from the clamping device, and this may lead to dangerous injuries.
It is a principal object underlying the invention to provide a clamping device of the above-mentioned kind to avoid springs difficult to dimension and clamping balls which cause damage to the tool shank, while assurance is given that on the one hand a sufficient clamping action is always exerted by the roll bodies onto the tool shank during the period the driving shaft is running, and that on the other hand the tool can freely be extracted from the clamping device when the driving shaft is stopped and stands still.